You state that your friend had his passport taken from him by the Gong An. The first thing worth mentioning is that the passport is not his property, but the property of his homeland's government. The second thing is if it was taken from him without his consent,that would be an illegal act. The third thing is that the authorities there seem to be treating him rather well - what I mean by that is he's still in China a month after his having overstayed his visa came to the attention of the Gong An. Simply for overstaying he could be fined 500 Yuan a day to a maximum of 5000 Yuan. He could also be 'requested to leave the country' within 10 days or less after being issued with the necessary short-term visa, which he would have to pay for.
His problem is further compounded by the fact he's been working without a Z visa. Another hefty fine could be in the pipeline for that. Is he still teaching? If I were him, I'd wire my family for the equivalent in his homeland's currency of 30000 Yuan, in readiness for paying fines, and buying a long-haul air ticket. The last thing he would want to do is time in a Chinese jail through non-payment of his fines or not having enough money left to pay for his flight!
That said, it would have been very easy for him to have checked the expiry date of his F visa, it must have
'valid until.....' written on it in English! Simply by having done that, he would surely not have ended up in so much trouble.